1. Field of the Invention
The present invention relates to monitoring contamination on semiconductor wafers and more particularly to the use of second-order nonlinear optics to determine the level of contamination with a high degree of specificity.
2. Description of the Related Art
In nonlinear optics, outputs are produced at sum, difference or harmonic frequencies of the input(s). Using second order nonlinear optical surface spectroscopy to examine the physical properties and behavior of a surface or interface was originally proposed in the 1960""s, in xe2x80x9cLight Waves at the Boundary of Nonlinear Mediaxe2x80x9d by Bloembergen and P. S. Pershan, The Physical Review, 128, Page 193 (1962). Experimental work involving second harmonic generation was also performed. However, because lasers at the time were comparatively feeble, impractical, slow, etc., there was little subsequent work done on the development of second harmonic generation or, more generally, second order nonlinear optical (NLO) processes at surfaces until considerably later.
Recently, researchers have reviewed NLO processing and concluded that lasers had developed enough that they could be used for studying the physical and chemical properties of surfaces and interfaces. For example, a theoretical study of the physics of the interface, and not its engineering aspects, has been performed. See Journal of Vacuum Science and Technology B, Volume 3, Number 5, September October 1985, Pages 1464-1466, Y. R. Shen, xe2x80x9cSurface Studies by Optical Second Harmonic Generation: an Overview.xe2x80x9d
In U.S. Pat. No. 5,294,289, T. F. Heinz et al. discuss the use of second harmonic generation as a means to monitor the epitaxial growth of silicon semiconductor structures in a high vacuum chamber. Specifically, they examined the spectroscopic response at the interface between the electronically active silicon and the insulative layer of calcium fluoride. By monitoring the magnitude of the resonance, they could ascertain whether the insulator was present on the surface and whether it had electronically binded to the underlying semiconductor. The system that is used examines the total intensity only of the second harmonic light that is generated and there is no discussion of calibration against other signals produced at the surface. There is also no discussion of the use of second harmonic generation (SHG) for the detection of contamination.
In U.S. Pat. No. 5,623,341, J. H. Hunt discusses the use of sum-frequency generation for the detection of contamination and corrosion on engine parts. In this incarnation, one of the inputs is a tunable IR beam that is tuned to a resonance of the contamination on the surface. The efficiency of the sum-frequency process is increased (so-called resonant enhancement) when the IR beam is resonant with a contaminant. If the contaminant is not present, there is no resonant enhancement. By comparing on and off resonant signals, the presence and level of contaminant can be deduced. However, there is no discussion of application to semiconductor materials. Given that the nonlinear optical response of metal and semiconductor are quite different, one cannot assume that the diagnostic is useful in the other environment.
In U.S. Pat. No. 5,875,029, P. C. Jann et al. describe a versatile optical inspection instrument and method to inspect magnetic disk surfaces for surface defects. The device provides surface position information of the defects. However, the technique involves only linear optical processes. That is, the input and output light wavelengths are the same. There is also no discussion of contamination.
In U.S. Pat. No. 5,883,714, Jann et al. describe a versatile optical inspection instrument and method to inspect magnetic disk surfaces for surface defects. The device is based on interferometric measurement and detects contaminants by measuring the Doppler shift in the light that results from scanning the light onto a contaminant or defect. By scanning, the device provides surface position information of the defects. However, the technique involves only linear optical processes and senses only phase changes. That is, the input and output light wavelengths are the same and there is no discussion of contamination.
In U.S. Pat. No. 5,898,499, J. L. Pressesky discusses a system for detecting local surface discontinuities in magnetic storage discs. The device is an interferometric detector which scans the disc in a spiral motion. Local defects cause local changes in phase which are measured by interferometric techniques. This is a linear optical technique.
In U.S. Pat. No. 5,932,423, T. Sawatari et al. discuss a scatterometer for detecting surface defects in semiconductor wafers. This device is a linear interferometric device.
In U.S. Pat. No. 5,973,778, J. H. Hunt discusses the use of second harmonic generation for investigating molecular alignment within a thin polyimide film. The technique uses changes in the second harmonic polarization to determine surface molecular alignment. There is no discussion of semiconductor materials, or contamination. The nonlinear optical response of a semiconductor will be quite different than that of a liquid crystal film.
In U.S. Pat. No. 6,317,514 B1, S. Reinhorn et al. discuss a method and apparatus for inspecting a wafer surface to detect the presence of conductive material on the wafer. The device uses UV initiated electron emission to determine the location of conductive areas. Those areas which are metal will emit electrons. If the area, which is supposed to be conductive, is not, there will be no electron emission.
In U.S. Pat. No. 6,359,451 B1, G. N. Wallmark discusses a system for testing for opens and shorts between conductor traces on a circuit board. The technique uses electron scattering to perform its diagnostics and has no optics associated with it.
In a broad aspect, the optical system of the present invention includes a first optical source for providing a first laser input directable to a location on a semiconductor wafer to be interrogated. A second optical source provides a second laser input directable to the semiconductor wafer location to be interrogated. The first and second laser inputs are alignable so that their surface locations of optical illumination overlap on the interrogated location. A first surface optical signal analyzer receives a first light signal at a first second-harmonic wavelength generated by the first laser input on the semiconductor wafer to be interrogated. The first surface optical signal analyzer converts the first light signal at the first second-harmonic wavelength to a first electronic signal, thus monitoring the intensity of the first second-harmonic wavelength, as a function of semiconductor wafer contamination. A second surface optical signal analyzer receives a second light signal at a second second-harmonic wavelength generated by the second laser input on the semiconductor wafer to be interrogated. The second surface optical signal analyzer converts the second light signal at the second second-harmonic wavelength to a second electronic signal, thus monitoring the intensity of the second second-harmonic wavelength, as a function of semiconductor wafer contamination. A third surface optical signal analyzer receives a third light signal at a sum-frequency wavelength generated by the first laser input and the second laser input on the semiconductor wafer to be interrogated. The third surface optical signal analyzer converts the third light signal at the sum-frequency wavelength to a third electronic signal, thus monitoring the intensity of the sum-frequency wavelength, as a function of semiconductor wafer contamination. An electronic signal analyzer compares the first, second and third electronic signals for determining the level of semiconductor wafer contamination.
Other objects, advantages, and novel features will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.